APIs and Middleware: Designing Shared Interfaces

APIs and middleware live at the boundary of your software system. They define how services talk to each other. Designing shared interfaces helps teams move faster, reduces surprises, and keeps systems maintainable. A good interface acts as a contract: clear, stable, and well documented.

What are shared interfaces?

  • They describe data shapes, operations, and error behavior.
  • They work across teams and deployment boundaries.
  • They stay compatible over time, or clearly announce changes.

Principles to follow:

  • Clarity: names, semantics, and required fields should be obvious.
  • Stability: avoid breaking changes; if needed, use a versioned path or header.
  • Observability: provide consistent tracing, metrics, and structured errors.
  • Security by default: enforce authentication and authorization at the boundary.
  • Interoperability: choose common formats (JSON, Protobuf) and standard protocols.

Patterns you can use:

  • REST with a formal contract, like OpenAPI, for wide compatibility.
  • gRPC or GraphQL for more opinionated or efficient communication.
  • Middleware layers for concerns: auth, rate limiting, retries, and logging before calls reach business logic.

Examples:

  • A user service interface might offer createUser, getUser, updateUser, and deleteUser with predictable field names and a stable response envelope.
  • A payment service might expose a single checkout call and a separate webhook channel for events, keeping those paths stable.

Common pitfalls:

  • Too many changes at once; communicate versioning and deprecation.
  • Hidden dependencies between teams; document them in the contract.
  • Ignoring observability; you cannot fix what you cannot measure.

How to start designing:

  • Define consumer needs, not just API endpoints.
  • Produce a shared contract early, then iterate with feedback.
  • Use gateways or adapters to enforce boundaries and keep services decoupled.

When done well, shared interfaces speed development, reduce bugs, and help your system scale.

Key Takeaways

  • Start with a clear contract and stable interface.
  • Document, version, and observe to manage changes.
  • Design for consumer needs and system decoupling.